Issue 62

I. Shardakov et alii, Frattura ed Integrità Strutturale, 62 (2022) 561-572; DOI: 10.3221/IGF-ESIS.62.38

D ESCRIPTION OF EXPERIMENTAL SETUP

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he photograph of the experimental setup is shown in Fig.3 It consists of a glass cylindrical vessel 1, on which a nichrome wire 6 is evenly wound to provide its heating and maintain the prescribed temperature. Air inside the vessel is stirred to ensure the uniform temperature distribution. In the process of stirring, the air is drawn in by the turbine 3 in the lower part of the vessel, and then it moves through the aluminum tube 7 into the chamber in the upper part of the vessel 8. After that, the air returns to the working area of the vessel through the perforated plate. From above the vessel is plugged with seal 9, which does not allow the air from the environment to get into the vessel. From below it is protected by insulating pad 2 to reduce a heat flux through the bottom. Inside the vessel there is an optical fiber with a Bragg grating 11. Fiber is loaded with a set of weights 4. Attachment to the fiber is by means of grip 5. Through the distributing plate 13 the fiber is clamped between rubber pads 14. Compressive force is produced by elastic clamp 12. To reduce the compressive force in the clamp 12 to a level that ensures no damage to the fiber, a ring element is provided in the grip 5. This reduction in clamping force is achieved due to the frictional forces generated by winding the fiber around the annular part. Changing the amount of winding makes it easy to adjust the axial force in the fiber with a constant compressive force in the clamp12. The temperature sensor 10 is brought to the fiber in the region of the Bragg grating 11 at a distance of 1-2 mm. To minimize the influence of the environment, during the experiment the vessel is placed inside a cube made of 50 mm thick foam plastic.

Figure 3: Experimental setup.

Power supply to the heating wire is regulated with a microcontroller and an external circuit on a field-effect transistor. The microcontroller is controlled by a computer, which measures temperature. A special computer program was developed to implement the PID regulation algorithm, which maintains the prescribed temperature inside the vessel and operates the step-wise temperature loading. The generation of a broadband optical signal and recording of the spectra reflected from the Bragg grating were accomplished using the ASTRO A322 interrogator manufactured by FiberSensing. The Bragg grating is fabricated in a single-mode fiber of the SM1500(9-125)P series. The initial grating wavelength is 1525 nm Temperature registration is performed using the B57861-S 103-F40 thermistor manufactured by EPCOS. The thermistor resistance is measured with a 24-bit Leonardo 2 ADC and a GSPF-052 generator manufactured by Rudnev-Shilyaev producer. Synchronization of

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